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1. Bounds on stomatal size can explain scaling with stomatal density in forest plants

   https://doi.org/10.1111/nph.70626  

   Liu, Congcong; Muir, Christopher D; Sack, Lawren; Li, Ying; Xu, Li; Li, Mingxu; Zhang, Jiahui; de_Boer, Hugo Jan; Han, Xingguo; Yu, Guirui; et al (December 2025, New Phytologist)

   Summary A prevailing hypothesis posits that achieving higher maximum rates of leaf carbon gain and water loss is constrained by geometry and/or selection to limit the allocation of epidermal area to stomata (f_S). Under this ‘stomatal‐area minimization hypothesis’, higherg_s,maxis associated with greater numbers of smaller stomata because this trait combination increasesg_s,maxwith minimal increase inf_S, leading to relative conservation off_Ssemi‐independent ofg_s,maxdue to coordination in stomatal size, density, and pore depth. An alternative hypothesis is that the evolution of higherg_s,maxcan be enabled by a greater epidermal area allocated to stomata, leading to positive covariation betweenf_Sandg_s,max; we call this the ‘stomatal‐area adaptation hypothesis’. Under this hypothesis, the interspecific scaling betweeng_s,max, stomatal density, and stomatal size is a by‐product of selection on a moving optimalg_s,max.We integrated biophysical and evolutionary quantitative genetic modeling with phylogenetic comparative analyses of a global data set of stomatal density and size from 2408 vascular forest species. The models present specific assumptions of both hypotheses and deduce predictions that can be evaluated with our empirical analyses of forest plants.There are three main results. First, neither the stomatal‐area minimization nor adaptation hypothesis is sufficient to be supported. Second, estimates of interspecific scaling from common regression methods cannot reliably distinguish between hypotheses when stomatal size is bounded. Third, we reconcile both hypotheses with the data by including an additional assumption that stomatal size is bounded by a wide range and under selection; we refer to this synthetic hypothesis as the ‘stomatal adaptation + bounded size’ hypothesis.This study advances our understanding of scaling between stomatal size and density by mathematically describing specific assumptions of competing hypotheses, demonstrating that existing hypotheses are inconsistent with observations, and reconciling these hypotheses with phylogenetic comparative analyses by postulating a synthetic model of selection ong_s,max,f_S, and stomatal size. 

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2. Leaf relative water content at 50% stomatal conductance measured by noninvasive NMR is linked to climate of origin in nine species of eucalypt

   https://doi.org/10.1111/pce.14700  

   Coleman, David; Windt, Carel W; Buckley, Thomas N; Merchant, Andrew (December 2023, Plant, Cell & Environment)

   Abstract Stomata are the gatekeepers of plant water use and must quickly respond to changes in plant water status to ensure plant survival under fluctuating environmental conditions. The mechanism for their closure is highly sensitive to disturbances in leaf water status, which makes isolating their response to declining water content difficult to characterise and to compare responses among species. Using a small‐scale non‐destructive nuclear magnetic resonance spectrometer as a leaf water content sensor, we measure the stomatal response to rapid induction of water deficit in the leaves of nine species of eucalypt from contrasting climates. We found a strong linear correlation between relative water content at 50% stomatal conductance (RWC_gs50) and mean annual temperature at the climate of origin of each species. We also show evidence for stomata to maintain control over water loss well below turgor loss point in species adapted to warmer climates and secondary increases in stomatal conductance despite declining water content. We propose that RWC_gs50is a promising trait to guide future investigations comparing stomatal responses to water deficit. It may provide a useful phenotyping trait to delineate tolerance and adaption to hot temperatures and high leaf‐to‐air vapour pressure deficits. 

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3. Sensitive Hydraulic and Stomatal Decline in Extreme Drought Tolerant Species of California Ceanothus

   https://doi.org/10.1111/pce.15208  

   Zailaa, Joseph; Trueba, Santiago; Browne, Marvin; Fletcher, Leila R; Buckley, Thomas N; Brodersen, Craig R; Scoffoni, Christine; Sack, Lawren (February 2025, Plant, Cell & Environment)

   ABSTRACT Identifying the physiological mechanisms by which plants are adapted to drought is critical to predict species responses to climate change. We measured the responses of leaf hydraulic and stomatal conductances (K_leafandg_s, respectively) to dehydration, and their association with anatomy, in seven species of CaliforniaCeanothusgrown in a common garden, including some of the most drought‐tolerant species in the semi‐arid flora. We tested for matching of maximum hydraulic supply and demand and quantified the role of decline ofK_leafin driving stomatal closure. AcrossCeanothusspecies, maximumK_leafandg_swere negatively correlated, and bothK_leafandg_sshowed steep declines with decreasing leaf water potential (i.e., a high sensitivity to dehydration). The leaf water potential at 50% decline ing_swas linked with a low ratio of maximum hydraulic supply to demand (i.e., maximumK_leaf:g_s). This sensitivity ofg_s, combined with low minimum epidermal conductance and water storage, could contribute to prolonged leaf survival under drought. The specialized anatomy of subg.Cerastesincludes trichomous stomatal crypts and pronounced hypodermis, and was associated with higher water use efficiency and water storage. Combining our data with comparative literature of other California species, species of subg. Cerastesshow traits associated with greater drought tolerance and reliance on leaf water storage relative to other California species. In addition to drought resistance mechanisms such as mechanical protection and resistance to embolism, drought avoidance mechanisms such as sensitive stomatal closure could contribute importantly to drought tolerance in dry‐climate adapted species. 

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4. Climate and hybridization shape stomatal trait evolution in Populus

   https://doi.org/10.1101/2025.07.21.665994  

   Zavala-Paez, Michelle; Keller, Stephen; Holliday, Jason; Fitzpatrick, Matthew C; Hamilton, Jill (July 2025, bioRxiv)

   Abstract Stomata play a critical role in regulating plant responses to climate. Where sister species differ in stomatal traits, interspecific gene flow can influence the evolutionary trajectory of trait variation, with consequences to adaptation.Leveraging six latitudinally-distributed transects spanning the natural hybrid zone betweenPopulus trichocarpa–P. balsamifera, we used whole genome resequencing and replicate common garden experiments to test the role that interspecific gene flow and selection play to stomatal trait evolution.While species-specific differences in the distribution of stomata persist betweenP. balsamiferaandP. trichocarpa, hybrids on average resembledP. trichocarpa. Admixture mapping identified several candidate genes associated with stomatal trait variation in hybrids includingTWIST, a homolog ofSPEECHLESSinArabidopsis, that initiates stomatal development via asymmetric cell divisions. Geographic clines revealed candidate genes deviating from genome-wide average patterns of introgression, suggesting restricted gene flow and the maintenance of adaptive differences. Climate associations, particularly with precipitation, indicated selection shapes local ancestry at candidate genes across transects.These results highlight the role of climate in shaping stomatal trait evolution inPopulusand demonstrate how interspecific gene flow creates novel genetic combinations that may enhance adaptive potential in changing environments. 

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5. Leaf Elementomes Reveal Close Links With Leaf Water‐Use Strategies Across Diverse Forest Ecosystems: Insights From Trait Coordination and Reflectance Spectroscopy

   https://doi.org/10.1111/pce.70323  

   Yang, Nan; Wu, Fengqi; Sack, Lawren; Querejeta, José Ignacio; Cernusak, Lucas A; Dong, Tingting; Xu, Weiying; Townsend, Philip A; Detto, Matteo; Peñuelas, Josep; et al (March 2026, Plant, Cell & Environment)

   ABSTRACT Leaf intrinsic water use efficiency (iWUE) quantifies the trade‐off between carbon assimilation and water loss in plants, and is constrained by leaf traits such as maximum carboxylation capacity (V_c,max) and stomatal conductance. Yet, the potential links of iWUE with leaf elementomes across different forest types remain unclear. Here, we analyzed iWUE (estimated by leaf carbon isotopes) variability and its associations withV_c,max, stomatal conductance (estimated by¹⁸O enrichment in leaf dry matter above source water, Δ¹⁸O), and leaf elementomes across 82 tree species from temperate, subtropical and tropical forests, and evaluated the effectiveness of leaf reflectance spectroscopy as an indicator of iWUE variability and trait–iWUE associations. Across species,V_c,max, Δ¹⁸O, leaf mass per area (LMA) and leaf iron, nitrogen, sodium and manganese concentrations were the traits most strongly associated with cross‐site iWUE variability. Furthermore, climatic factors (mean annual precipitation, mean annual temperature and climate moisture index) shaped trait–iWUE covariation by negatively linking leaf elements and positively with LMA, which affected iWUE more directly than indirectly viaV_c,maxand Δ¹⁸O. Leaf reflectance spectroscopy accurately predicted iWUE (R² = 0.83), and the trait–iWUE relationships derived from spectral modelling were consistent with those obtained through field measurements. These findings reveal strong linkages between the leaf elementomes and iWUE, and highlight the potential of reflectance spectroscopy for characterizing iWUE variability and trait–iWUE relationships, thereby improving process modelling of forest carbon and water cycles. 

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